As an approach for increasing the side lighting brightness of a non-directional side lighting type plastic optical fiber, a method has been known in which the side lighting brightness and the decay rate of side lighting brightness is controlled by adding a minute amount of titanium oxide to the clad and varying the content of titanium oxide (see, e.g., Japanese Patent No. 3384396, published on Mar. 10, 2003).
As another approach for increasing the side lighting brightness of a non-directional side lighting type plastic optical fiber, a method has been known in which the side lighting brightness is controlled by adding light scattering agent to the core and varying the content of light scattering agent (see, e.g., Japanese Patent Publication No. 2000-131529, published on May 12, 2000).
However, when titanium oxide is added to the clad as in the optical fiber, if separation of layers occurs at the core/clad interface due to shock or the like, light scattering is reduced at the separated region, which gives rise to the appearance of so-called “dark defect”, leading to impairment of the external appearance of the fiber.
On the other hand, when light scattering agent is added to the core as in the optical fiber, since light scattering takes place not at the core/clad interface but in the core, layer separation at the core/clad interface is unlikely to give rise to the appearance of “dark defect”, and the fiber appearance can be maintained in good condition. However, with this optical fiber, since the clad is free of light scattering agent and is transparent, if scratch is produced on the clad surface, the scratch may strongly scatter light, giving rise to so-called “bright spot”, which again leads to impairment of the external appearance of the fiber.